Does Firefox Sync Encrypt AI Extension Data Stored Locally On Disk?

August 30, 2026

Firefox Sync encrypts synchronized data before it leaves the browser, but Mozilla says WebExtension storage itself is not encrypted.

No. Firefox Sync's end-to-end encryption protects synchronized data before it leaves Firefox and while Mozilla's servers hold it, but that does not mean an AI extension's copy is encrypted in the local Firefox profile. Mozilla's WebExtensions documentation says the extension storage area is not encrypted and should not be used for confidential information.

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That distinction matters because an AI extension may handle prompts, selected page text, summaries, notes, drafts, API settings, or identifiers. A protected remote Sync copy and a readable local extension store can exist at the same time.

The practical rule is simple: treat Sync encryption, local-disk protection, the extension's own storage design, and any developer-operated cloud service as four separate security boundaries.

Who This Guide Is For

This guide is for people who use a Firefox extension to:

It is also for extension developers deciding whether storage.local or storage.sync is an appropriate place for user content.

The Short Answer In One Table

Copy or boundary What Mozilla documents What that does not prove
Firefox Sync server copy Sync data is encrypted before leaving the browser and can be decrypted by another signed-in Firefox instance That the local Firefox profile is encrypted at the extension-storage layer
storage.sync local copy Firefox keeps synchronized extension state in the browser profile and makes it usable by the extension That a person or process with profile access cannot read the stored values
storage.local copy Data remains on the machine and can persist independently of normal site-history clearing That Firefox Sync protects it or that clearing site data deletes it
Extension's own cloud account Controlled by the extension developer's architecture, policy, and deletion tools That Firefox or Mozilla governs retention, training, access, or deletion
Operating-system drive Full-disk encryption can protect files from offline access when the device is locked or powered off That malware, another process, or an unlocked user session cannot access data
Backup or old profile May contain a copy of the Firefox profile from an earlier time That deleting the active extension record removed historical copies

What Is Confirmed

Mozilla's current documentation confirms the core split between remote Sync encryption and local WebExtension storage.

Firefox Sync is end-to-end encrypted

Mozilla's Firefox Sync guide says Sync encrypts data before it leaves the browser. Mozilla receives an encrypted server-side copy, and another Firefox instance with the necessary account-derived key can decrypt it.

Mozilla's separate explanation of how Sync remains protected even if TLS fails says the encryption key is not sent to Mozilla's Sync server. That is meaningful protection for the synchronized copy in transit and at Mozilla.

But end-to-end encryption describes the path between trusted endpoints. Each endpoint still has to make the data available to Firefox and the extension after the user signs in. The endpoint is not automatically an opaque vault just because the network and server copy are encrypted.

Mozilla says WebExtension storage is not encrypted

MDN's current WebExtensions storage documentation states that the storage area is not encrypted and should not be used for confidential user information.

The same page divides extension storage into several areas:

The word sync therefore describes synchronization behavior. It is not a promise that the local values are stored as unreadable ciphertext inside the Firefox profile.

Firefox keeps profile data separately from the application

Mozilla's Firefox profile guide explains that bookmarks, history, extensions, settings, sessions, and other browser data live in a profile folder separate from the Firefox program.

On Windows, the default roaming Firefox profile is inside the user's AppData directory. Other operating systems use different profile locations. The important point is not the exact folder name. It is that reinstalling Firefox, backing up a profile, copying a user profile, or restoring an old profile can preserve browser data separately from the browser executable.

Mozilla's profile backup guidance also says that Firefox Refresh can place an old profile backup on the desktop. A user who deletes data from the active profile may still have an older copy in a backup, system snapshot, migration archive, or another device.

storage.sync creates a local endpoint copy

MDN's storage.sync reference says synchronized extension values become available across signed-in browser instances. On desktop Firefox, the user must select Add-ons in Sync, and current Mozilla documentation says Firefox for Android does not synchronize this extension data.

Firefox compares the local and server values for each key during synchronization. A newer server value can be written into the browser's local sync storage. That behavior would not work if the browser could never recover a usable local value for the extension.

This is why a prior OpenVeil guide asks whether Firefox Sync can overwrite AI extension data from another device. Synchronization protects and distributes a record; it is not the same operation as locally encrypting every extension value against access from the signed-in device.

storage.local can survive ordinary browsing-data cleanup

MDN's storage.local reference says values in this area are local to the machine. Mozilla also distinguishes the WebExtensions storage API from extension use of ordinary localStorage: Firefox may clear the latter in some browsing-data cleanup scenarios while correctly persisting data stored through storage.local.

So clearing website cookies, history, or an AI site's origin storage is not proof that an extension-controlled copy disappeared. A Firefox extension can have its own storage boundary outside the website's database.

This is also why browser sync usually is not a backup of browser-local AI chat history. Website storage, browser history, tab sync, extension storage, and an app-level cloud account are different records.

A Primary Password protects saved logins, not generic extension data

Mozilla's Primary Password documentation describes a specific protection for Firefox's saved usernames and passwords. Firefox stores saved logins in logins.json and the relevant key material in key4.db; a Primary Password adds protection to that password-manager key.

Mozilla does not describe Primary Password as a general encryption layer for an extension's storage.local, storage.sync, IndexedDB, downloads, cache, logs, or exported files. Turning it on is valuable for saved logins, but it should not be presented as a vault for every record in a Firefox profile.

What Is Still Unclear

Firefox documentation cannot tell you what a particular AI extension actually saves. That requires evidence from the extension itself.

For each extension, the unresolved questions include:

The statement “Firefox Sync is encrypted” answers none of those extension-specific questions by itself.

A Concrete Example

Imagine an extension stores two keys:

{
  "preferredModel": "example-model",
  "lastDraft": "Acquisition plan for Client Northwind..."
}

If the extension writes both values to storage.sync, Firefox can protect the synchronized payload before it leaves the browser. Mozilla's server receives an encrypted copy.

On a signed-in laptop, however, the extension needs the usable value of lastDraft to restore the draft. MDN's warning means the developer should not assume Firefox's extension storage itself is a confidential local vault.

If the extension instead encrypts lastDraft before calling the storage API, the stored value might look like application ciphertext. That can improve local protection only if the encryption key is handled separately and securely. Saving the key beside the ciphertext, hardcoding it in the extension, or automatically unlocking it for every local process can erase much of the benefit.

The preference is a reasonable Sync candidate. The confidential acquisition draft is not, unless the extension has designed and documented a stronger protection model.

What Full-Disk Encryption Helps With

Operating-system drive encryption is an important compensating control for local profile data.

Microsoft's BitLocker overview explains that drive encryption protects data from offline access, such as when someone removes a drive from a lost or stolen computer. Windows Device Encryption uses BitLocker technology on supported systems.

This can protect the entire Firefox profile, including extension storage, when the drive is locked and the decryption key is unavailable.

It does not make the extension store confidential in every situation. After the user unlocks the device and signs in, authorized software can normally read files through the operating system. Full-disk encryption does not stop:

Disk encryption protects a device state. It does not replace extension permissions, data minimization, application-level encryption, endpoint security, or provider governance.

How To Audit An AI Extension With The DISK Method

Use this four-part audit before storing sensitive material in a Firefox extension.

D — Discover every data store

List every place the extension may create a record:

Do not infer the answer from the presence of a Sync icon. Read the extension's documentation, privacy notice, permissions, and account settings.

For technical inspection, MDN says extension storage can be examined through the Extension Storage section of Firefox's developer tools opened from about:debugging. Use synthetic test data rather than a real secret. Developer tools can reveal what the current profile exposes, but they do not inventory remote services or historical backups.

I — Inspect what actually crosses each boundary

Create a distinctive but invented canary, such as OV-EXTENSION-TEST-4837, and save it through the feature being evaluated.

Then check:

  1. whether it appears in local extension storage;
  2. whether it reaches a second signed-in desktop browser;
  3. whether it appears in an extension account, provider dashboard, export, or log;
  4. whether it remains after clearing the AI site's data; and
  5. whether the extension's own delete control removes it everywhere you can inspect.

This test is more reliable than assuming that installing the same add-on means its data is synchronized. As the related OpenVeil guide explains, a Firefox extension can sync private AI data only when its design and settings create that path.

S — Secure the endpoint and its copies

Use a separate operating-system account when a device is shared. Enable supported full-disk encryption, use a strong device sign-in, keep Firefox and extensions updated, and review which extensions are installed.

Treat backups as copies of the protected data, not as invisible infrastructure. Encrypt backup media appropriately, control recovery keys, and know whether a cloud backup provider can access the decrypted content.

If an extension offers its own content-encryption setting, verify what it protects, where the key lives, and whether recovery creates another readable copy. Do not equate a lock icon with a complete threat model.

K — Keep confidential content out unless the design supports it

Mozilla's own MDN warning is the safest default: do not put confidential user information in the unencrypted WebExtensions storage area.

For a sensitive task, prefer one of these options:

Deletion Is A Separate Test

Encryption answers who can read a copy under certain conditions. Deletion answers whether the copy still exists. They are not interchangeable.

If you need to remove sensitive AI extension data, check separately:

  1. the active browser profile;
  2. every synchronized desktop Firefox profile;
  3. the extension's own cloud account;
  4. the AI provider's chat, file, feedback, and retention controls;
  5. downloads and exports;
  6. profile backups and operating-system snapshots; and
  7. old devices that have not synchronized the deletion.

Deleting a local key can later be undone by synchronization conflict or restoration from a backup. Conversely, deleting a Sync server record does not prove that another device, extension service, or provider copy disappeared.

Test deletion with synthetic data first. Confirm the canary is gone from each known store, restart the browser, force a Sync cycle, and recheck before treating the process as reliable.

What This Means For Browser-Local AI Privacy

“Browser-local” describes location, not automatic encryption. A record can be local to one browser profile and still be readable to software or people with access to that profile.

The OpenVeil guide to what browser-local chat history means makes the same distinction for AI apps: local history can avoid a normal server-side chat-history record, but it can be lost or exposed through browser data, profiles, device access, and backups.

Likewise, encryption does not make an AI chat private by itself. Privacy also depends on what is collected, which parties process it, how long copies remain, who can access them, whether content is reused, and whether deletion works.

The correct claim is narrow:

Where OpenVeil Fits

OpenVeil is an 18+ privacy-focused hosted AI workspace with browser-local normal chat history and no normal server-side chat-history record. It supports chat, web search, file workflows, voice, images, video, and custom personas without requiring a page-reading Firefox extension for ordinary use.

That narrower workflow can be useful when you want private AI chat but do not need an extension to observe email, documents, or every page you visit.

OpenVeil is not fully offline or anonymous. Active requests still require processing by OpenVeil and necessary AI, search, file-processing, hosting, routing, security, billing, and infrastructure providers. OpenVeil also does not encrypt unrelated Firefox extension storage, secure a compromised device, inspect third-party add-ons, or remove copies held by another provider.

The honest benefit is scope: if the task does not require a browser extension, avoiding that extra read-and-store boundary can make the workflow easier to understand and audit. OpenVeil offers a one-time ten-action preview with no card, and paid plans start at $10 per month.

Frequently Asked Questions

Does Firefox Sync encrypt extension data on Mozilla's servers?

Mozilla says Firefox Sync data is end-to-end encrypted before it leaves the browser, so Mozilla receives an encrypted server-side copy. That protection does not establish local extension-storage encryption.

Is storage.sync encrypted on my hard drive?

Do not assume so. MDN says the WebExtensions storage area is not encrypted and should not be used for confidential information. sync identifies synchronization behavior, not a local confidential vault.

Is storage.local protected by Firefox Sync?

No. storage.local is local to the machine. It is not the same store as storage.sync, and ordinary Firefox Sync does not turn every local extension record into an encrypted backup.

Does Firefox Primary Password encrypt AI extension data?

Mozilla documents Primary Password as protection for saved Firefox logins and their key material. It is not documented as general encryption for WebExtension storage, IndexedDB, downloads, or the whole profile.

Does BitLocker make AI extension storage safe?

BitLocker or Windows Device Encryption can protect the drive from offline access when the key is unavailable. It does not stop access from an unlocked session, malware, the extension itself, a cloud service, or an unencrypted backup.

Can another signed-in Firefox decrypt synchronized extension data?

That is the purpose of Sync: an authorized Firefox endpoint can recover and use synchronized data. This is why end-to-end encryption protects the route and server copy but does not make the endpoint unable to read its own data.

Does uninstalling an extension delete every copy?

Do not assume it deletes synchronized records on other devices, developer-operated cloud data, AI-provider records, exports, or profile backups. Audit each store under its own deletion rules.

Can an extension encrypt its own data before using Firefox storage?

Yes, an extension can implement application-level encryption. Its real protection depends on key storage, unlock behavior, recovery, logging, backups, and whether plaintext is exposed elsewhere in the workflow.

What should extension developers store in storage.sync?

Mozilla describes the main use as preferences and other compact shared state. Keep confidential prompts, drafts, and document content out unless the product has a documented protection model designed for those records.

Bottom Line

Firefox Sync can end-to-end encrypt synchronized data before it leaves the browser, but it does not automatically encrypt an AI extension's local profile data. Mozilla explicitly warns that the WebExtensions storage area is not encrypted and should not hold confidential information.

Use the DISK audit: discover every store, inspect the real data path with a synthetic canary, secure the endpoint and backups, and keep confidential content out unless the extension's design supports it.

The most important question is not “Is Sync encrypted?” It is: which copy are you trying to protect, from whom, while the device is in what state, and which component holds the key?

Sources

Published August 30, 2026. Browser behavior, extension implementations, and provider policies can change; verify the current documentation and test the exact extension, Firefox profile, device, backup, and account before relying on an encryption or deletion claim.

When privacy, account control, uploads, and search matter, OpenVeil gives you a private AI workspace designed for that job.